1. Field of the Invention
The present invention generally relates to eye tracking method and apparatus and, more particularly, to an improved eye tracking system having particular application for the measurement of saccadic eye movements.
2. Description of the Prior Art
An example of an ophthalmological instrument that implements an eye tracking mechanism for data acquisition is disclosed in U.S. Pat. No. 5,196,872 to Beesmer et al. This instrument requires an operator who, working with a subject, positions images of the subject's pupils on a display screen prior to performing a series of tests. During these tests, the subject's eyes remain relatively stationary, although there will be some small involuntary movement. The tracking system therefore needs to have only a limited response speed and, in fact, the response speed of the tracking system, incorporating as it does the video camera which images the subject's pupils, is limited by the video scanning rate of 60 Hz. This is quite adequate for the ophthalmological instrument disclosed in the Beesmer et al. patent; however, the response speed of that tracking system is not sufficient where fast eye movements are to be measured.
When a person looks from one place to another, the eyes undergo extremely fast rotations (up to 600.degree. per second), called saccadic eye movements, or saccades. It is useful for some applications to track the eye even when it is moving at the very high speeds of saccadic eye movements. For example, if the eye is to be tracked to measure some property like pupil diameter as in the ophthalmological instrument disclosed in the Beesmer et al. patent, a saccade can cause loss of tracking. While there are some video-based instruments that purport to measure saccadic eye movements, what they in fact measure is the presence of a movement faster than they can track. When such a movement is detected, these instruments declare the movement a saccade, and from this information, simply estimate the number of saccadic movements and when they occurred. If it is desirable to know where a person's eye is pointed right after a saccade, e.g., where the person is looking, the inability to track the eye at such high speeds results in loss of this information. It may also be desirable to know about the properties of the saccade itself, as in cases where the saccadic velocity is a significant variable to measure.
Because saccades are so quick (even a large one only lasts 50 or so milliseconds), they cannot be measured accurately unless the position of the eye is sampled at a high rate. Ordinary video rates (i.e., 17 or 34 msec between samples) do not give accurate readings of onset time, velocity, amplitude, shape, etc. Some special video systems having frame rates up to 400 Hz have high enough sampling rates, but these systems are very expensive and require high light levels that are not compatible with tracking the eyes. Therefore, it is desirable to sense eye positions with a photodetector which may be sampled at very high rates yet requires relatively low light levels so as to be compatible with an ophthalmological instrument.